On January 26, 2012, the United States Court of Appeals for the District of Columbia  unanimously upheld the Environmental Protection Agency's finding that carbon dioxide is a threat to public health and the environment. 

...human beings exhale carbon dioxide...

A few years after this ruling, it was
determined by the top minds in the country's elite institutions that everyone should be required to wear a specialized breathing mask that would minimize the amount of "carbon" emitted by someone outside.  Every building would be fitted with an air-filtration system that allow people to spend time indoors without
being required to wear the mask. 
While personally-owned vehicles were not banned outright, various "sin taxes" were placed not only on the sale of new vehicles, but also on the sale of gasoline.  Trains are now the primary mode of transportation, despite being notoriously late and overcrowded. 
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